Have you tested your water? With that small of a tank, and that large of a bioload, your tank is prone to sudden spikes in ammonia/nitrite/nitrate. Only 2 neons? They do best in a group of at least 5 or 6. My suggestion is to try and get a bigger tank...or return some fish or give them to friends if they have aquariums until you can get a bigger tank.

Sometimes, sluggishness can be linked to disease. Any other symptoms with the fish?